In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of radio frequency (RF) current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in a high-pressure gaseous lamp fill comprising the combination of a metal halide and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by RF current in an excitation coil surrounding an arc tube which contains the fill. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a changing magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, thus producing a toroidal arc discharge in the arc tube.
To maximize efficiency of an HID lamp, the coefficient of electromagnetic coupling between the excitation coil and the solenoidal discharge must be maximized. Since the degree of coupling increases with frequency, electronic ballasts used to drive HID lamps operate at high frequencies in the range from 0.1-20 MHz, exemplary operating frequencies being 13.56 and 6.78 MHz. These exemplary frequencies are within the industrial, scientific, and medical band of the electromagnetic spectrum in which moderate amounts of electromagnetic radiation are permissible; and such radiation is generally emitted by an electrodeless HID lamp system.
A suitable electrodeless HID lamp ballast is described in commonly assigned of J. C. Borowiec and S. A. El-Hamamsy, copending U.S. patent application 472,144, filed Jan. 30, 1990, which patent application is incorporated by reference herein. Operation of an electrodeless HID lamp ballast at the resonant frequency of the ballast load circuit maximizes power output, while operation at a frequency slightly lower than the resonant frequency of the load circuit maximizes ballast efficiency. Hence, for maximum efficiency, operation is slightly "off" resonance, and a specific ballast load amplitude and phase angle are required. To this end, the impedance of the ballast load, including that of the arc discharge as reflected into the ballast load, must be matched to the required ballast load resistance and phase angle. As described in U.S. patent application Ser. No. 472,144, cited hereinabove, a capacitance connected in parallel with the excitation coil is needed to match the resistive component of the ballast load impedance, and a capacitance connected in series with the excitation coil is needed to obtain the proper phase angle.
To meet the relatively large current requirements of an electronic HID lamp ballast using standard RF capacitors, such as multilayered ceramic capacitors and RF transmission capacitors, several standard-valued RF capacitors must be connected in parallel. Such a configuration is usually bulky. In addition, as the number of ballast circuit elements increases, the number of electrical leads and connections increases, resulting in more resistive losses. Moreover, electrical leads have parasitic inductances associated therewith which may introduce additional resonances into the ballast load circuit. An additional resonance resulting from a parasitic inductance introduces waveform distortion and increases power dissipation, thereby reducing efficiency.